The International System of Units (SI) is the world's most widely used system of measurement, both in everyday commerce and in science.

The International System of Units (abbreviated SI from the French Le Système International d'Unités) is the modern form of the metric system. It is the world's most widely used system of measurement, both in everyday commerce and in science.

The six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities are: metre, kilogram, second, ampere, kelvin, mole, and candela.

Flow specifications
Flow is specified in many different units, and some of these are not explicit. The SI unit of flow is m3/s, but it very popular to refer to flow in m3/h. And small flows may be specified as liters/minute or liters /hour.

Oilfield applications may specify flow in barrels.

Deviations in measurement
When old fashioned imperial units are used, the gallon is very popular. The problem with gallon is that there are two different gallons:

The original imperial gallon (UK) is about 277,418 cubic inches or 4.546 liters.

The Americans use a US gallon which is 231 cubic inches or 3.785 liters.